Mixin Does not throw issue when implementing interface

[richard457]

abstract class C {
FutureOr<String?> e();
FutureOr<int?> d();
}

// if we do not implement methods defined in C, the mixin will not throw error 
mixin AMixin implements C {

}

[FMorschel]

From my understanding, this is working as intended. @lrhn or someone from the language team can confirm.

IIRC, this is because a mixin can't be instantiated by itself, it always needs a class. The class that adds it or implements it should then implement that interface from C too.

[eernstg]

True, a mixin does not have to have an implementation of every member in its interface. Similarly, when a mixin is applied to a class it is also not necessary for the resulting class to have an implementation of every member in its interface (that is, a mixin application yields an abstract class). However, as soon as these abstract classes are used as a superclass of a concrete class there must be an implementation of every member of the interface.

It could make sense to say that a specific mixin is intended to provide an implementation for every method in a given interface that it implements, so you might want to have a warning in the case where some members are not implemented by the mixin. There is no direct support for that. However, you can check this by creating a concrete mixin application class that just applies the mixin to its on type:

import 'dart:async';

abstract class C {
  FutureOr<String?> e();
  FutureOr<int?> d();
}

mixin AMixin implements C {
  FutureOr<String?> e() async => "Hello!";
}

class _TestAMixinConcreteness = Object with AMixin; // Error: `d` not implemented.

[eernstg]

I'll close this issue: It is all working as intended.